When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via a transvenous leadwire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
One aspect of typical cardiac rhythm management systems is providing an appropriate endocardial lead coupling an implantable cardiac rhythm management device to the heart for sensing intrinsic electrical heart activity signals and/or delivering electrical therapy such as pacing stimulations or defibrillation countershocks. In conventional cardiac rhythm management systems, the lead typically includes a distal electrode that is transvenously guided into the right atrium or right ventricle. The electrode is carefully positioned to contact adjacent cardiac tissue for sensing and/or providing therapy. The preferred location provides adequate amplitude sensed intrinsic electrical heart activity signals and low threshold energy requirements for effectively delivering pacing and/or defibrillation therapy. To ensure that the electrode's location does not change over time in spite of continuous heart contractions, a fixation device may be used to anchor the distal end of the leadwire in position within the heart.
Even if the position of the electrode is stabilized, stimulation thresholds may vary over time as a result of interactions between the electrode and the adjacent tissue. Fibrotic scar tissue may form during the recovery and healing process as the body reacts to the presence of the electrode. The growth of fibrotic tissue results in chronic stimulation energy thresholds that exceed the acute energy thresholds obtained immediately after implant. As a result, higher stimulation energies are required, thereby shortening the usable life of the battery-powered implantable cardiac rhythm management device. By providing a steroid at the electrode and/or its associated fixation device, modified fibrotic tissue is formed and lower chronic stimulation thresholds are obtained, as disclosed in Heil, Jr. et al., U.S. Pat. No. 4,819,661, which is assigned to the assignee of the present application, and which is incorporated herein by reference in its entirety.
Use of a fixation device to anchor the lead typically obtains consistent chronic sensing and stimulation thresholds, but such fixation devices complicate removal of the lead such as, for example, in the event of an infection or lead failure. For example, removing a lead with a corkscrew or barb fixation device risks damage to the surrounding tissue to which the fixation device is firmly attached. Where such surrounding tissue is fibrotic scar tissue resulting from lead placement and fixation in conjunction with the normal healing process, separation of the lead from the surrounding scar tissue is even more difficult because the lead will likely have become firmly incorporated within the body. Moreover, many such fixation devices are adapted only for securing the distal tip of the lead; they may not adequately secure ring electrodes that are disposed at a slight distance away from the distal tip of the lead. Furthermore, most conventional fixation techniques are directed toward anchoring leads in the right side of the heart. Such techniques may not be as well suited for anchoring leads elsewhere. Thus, there is a need for providing a cardiac rhythm management lead that is capable of both adequate fixation and removal. There is also a need for providing a cardiac rhythm management lead that is capable of placement and fixation in other regions of the heart, such as within fragile vascular structures.